Essentially, a centrifuge is a device for separating particles suspended in a sample solution. A centrifuge rotor that contains the sample solution is driven at high rotational speeds inside an enclosed chamber. Typically, the chamber contains air at atmospheric pressure, but it is not uncommon to operate a centrifuge system at less than atmospheric pressure. The reduction in pressure decreases windage power consumption. In an extreme instance, ultracentrifuges are operated at high vacuums to reduce frictional heating of the rotor. Typically, high speed laboratory centrifuges are operated in a range having a high end of 1 atmosphere and a low end of 0.5 atmosphere, but in special applications gases such as helium, nitrogen and argon may be substituted for air and the high end of the range may exceed 1 atmosphere.
A centrifuge rotor is heated to a minor extent by thermal conduction from a drive motor, through a drive shaft. However, in instances other than use of the ultracentrifuge, heating of a high speed rotor occurs primarily by thermal conduction from the air or other gas within the chamber, with the gas being heated by the work done on the gas by the rotor. This work takes the form of accelerating the gas and inducing a pumping action that then leads to rapid recirculation of the gas and a buildup of heat. It is known to provide such centrifuges with refrigeration systems designed to extract heat from the chamber in order to maintain the rotor at a desired temperature.
One of the problems encountered in the design of high speed laboratory centrifuges relates to the requirement that the centrifuge be operable with numerous interchangeable rotors. In some circumstances, there are as many as twenty different interchangeable rotors for a general-purpose, high speed laboratory centrifuge. Rotor models are made in a range of sizes and have numerous variations in design. Each rotor model has a rated maximum safe rotational speed, which generally depends on maximum allowable centrifugally induced stresses. The rated maximum safe speeds cover a large range. To accommodate the speed requirements, a centrifuge drive system is provided with a wide range of adjustability. However, the various rotors will differ considerably in the windage power consumed when the rotors are spun at the maximum speed. It follows that the refrigeration power required to neutralize the heating of the air in the chamber will depend on the specific rotor and on the speed at which the rotor is operated. In prior art designs that include refrigeration systems, the temperature of the enclosed chamber is typically monitored. For example, the air flowing slightly above the bottom of the chamber may be temperature monitored. More or less satisfactory control has been obtained by experimentally determining the optimal refrigeration settings for the individual rotors over a range of speeds. Thus, it is necessary to select settings designed for a desired rotor and, additionally, provide special offsets for some rotors depending upon exact calibration of the rotor versus refrigeration settings.
The reasons for the difficulty of determining and setting the refrigeration controls derive from the physical laws associated with the aerodynamics of rotating bodies. The equation describing windage power sets forth the power losses as being proportional to the cube of the rotational speed and to the fifth power of the diameter of the rotating body when the rotating body is in a relatively close fitting and smooth chamber. As the chamber walls are moved proportionally further from the rotating body, the windage losses increase considerably from that predicted by the simple equation. Thus, both for proper temperature control and for safety against rotor failure through accidental overspeed setting, the rotor must be correctly identified.
It has been customary to depend upon the operator to correctly identify the rotor and adjust speed and refrigeration settings accordingly. More recently, there has been a growing concern and requirement for safety redundancy in rotor identification and, even in the instance of ultracentrifuges which have had at least one level of overspeed protection for years, an additional level has been introduced. In many instances, the secondary and tertiary identification need not be absolute, since it is sufficient to provide differentiation to the extent that no rotor is spun at a speed higher than its rated maximum safe speed. Several quite different rotors may have identical allowable speeds, and the only requirement is that the secondary and tertiary identification differentiate these rotors from all rotors that have a higher rated maximum safe speed.
Indicative of a redundant identification system is the apparatus described in U.S. Pat. No. 4,827,197 to Giebeler, which is assigned to the assignee of the present invention. Giebeler teaches that an identification of a rotor may be made by calculating the moment of inertia of the rotor. The rotor is accelerated under constant torque. Acceleration from a first speed to a second speed is timed and the moment of inertia is computed by using the calculations of change in speed and change in time. After obtaining the moment of inertia, Giebeler teaches that the identification can be made by matching the calculated moment of inertia to a known moment of inertia of one of a variety of different rotor models.
U.S. Pat. No. 5,235,864 to Rosselli et al. also teaches a redundant rotor identification system. However, instead of calculating moment of inertia, Rosselli et al. teaches measuring "windage," which is defined in the patent as being the resistance to rotor motion that is a result of fluid frictional effect. It is taught that "windage" is determined by either measuring the time needed to accelerate the rotor from a first relatively high speed to a second higher speed or measuring the change in speed that takes place within a preselected period of time. The resulting velocity signal or time signal generated during this step is then used to generate a rotor identity signal by means of either comparing the signal with a reference signal indicative of a reference "windage" value or by means of addressing a look-up table of "windage" values. It is taught that, in one embodiment, a preliminary decision is made as to whether the rotor lies in the high windage regime or the low windage regime of rotors. However, it is left unclear as to how the decision is to be based. In any embodiment, the determination of windage is achieved by accelerating the rotor at relatively high speeds at which Rosselli et al. teaches that windage becomes dominant to inertia in resisting acceleration of the motor.
One difficulty with the approach described in Rosselli et al. is that the generated velocity signal or time signal used to identify the rotor is responsive to both a windage component of rotor resistance and an inertia component. That is, the acceleration does not isolate components of resistance to rotor acceleration. Rosselli et al. teaches that the acceleration is to occur for speeds at which the windage component is dominant to the inertial component. However, the inertial component is present for any acceleration. Another concern with the approach of Rosselli et al. is that "windage" is defined as merely the resistance to motion resulting from a fluid frictional effect. As defined herein, "windage" is primarily the power consumed in pumping the gaseous atmosphere within the enclosed chamber of the centrifuge when the rotor is spun at high rotational speeds. At these high speeds, viscous frictional drag plays the role of providing mechanical coupling of the rotor to the mass of gas, resulting in the gas pumping. However, distinguishing viscous frictional drag and power consumed in pumping the gaseous atmosphere is important.
It is an object of the present invention to provide a method and system for assuring that any rotor from a family of rotors operable within a centrifuge will not be driven beyond a maximum safe operating speed for the rotor. A further object is to provide rotor operating information to a refrigeration control system, wherein the information is specific to the identified rotor.